Our long-term objectives are to elucidate the difference between the host-parasite interactions of human infective and human non-infective trypanosomes of the T. brucei species, in order to understand the mechanism of resistance to lysis by human serum. T. b. brucei cannot infect humans because it is lysed by normal human serum, while T. b. rhodesiense is a human pathogen because it is resistant to lysis. We now know there are two discrete trypanosome lytic factors (TLFs) in human serum with distinct biochemical properties. We have directed our attention to the analysis of the putative receptors for the TLFs and the mechanism of lysis. Our specific aims are 1) elucidation of the mechanism of lysis by human serum in order to understand the mechanism by which T. b. rhodesiense is resistant to lysis, 2) to evaluate the fate of TLF1 and TLF2 following their interaction with serum-resistant and serum-sensitive trypanosomes, and 3) characterization and molecular cloning of the TLF1 and TLF2 receptors in trypanosomes. The hypotheses to be tested are as follows. 1) TLFs bind to a lipoprotein scavenger receptor that selectively acquires lipids to supply the trypanosome's needs. 2) Resistance to TLF is due to differential routing and processing of the internalized TLF particle within the resistant parasite. 3) TLF mediated lysis is initiated by membrane lesions that cause an osmotic imbalance and activation of a cytosolic protease. Since resistance to lysis by human serum appears to be the critical characteristic that allows trypanosomes to successfully infect humans, it follows that understanding the nature of TLF1 and TLF2 and the mechanism by which trypanosomes are either sensitive or resistant is central towards revealing possible avenues for therapeutic intervention. We have preliminary evidence that trypanosomes may use a lipoprotein scavenger receptor to fulfill their obligate need for lipid uptake. We have shown that this putative receptor, which can facilitate the uptake of HDL and LDL, appears to be responsible for uptake of TLFs. These findings provide the insight and reagents to purify and/or clone this scavenger receptor, which would be only the second receptor to be molecularly cloned from trypanosomes. In addition we have the first indication that TLF may be able to form an ion channel, or modify an existing channel, and thereby facilitate the flux of ions into trypanosomes to initiate lysis. [unreadable] [unreadable]